In an electric power steering device that is mounted in a vehicle, such as a car, and that includes a steering assist motor, for example, if required steering assist power increases, it is necessary to provide a larger current to the steering assist motor. Therefore, in a case of parking a vehicle in a garage or in a parking lot, for example, where steering wheel operations are performed by the driver a number of times and power required by the steering assist motor rapidly increases, power corresponding to the required power might not be supplied by only an onboard battery (main power source).
In order to respond to such a case, Japanese Unexamined Patent Application Publication No. 2012-162210 (hereinafter referred to as PTL 1), for example, proposes a technique in which, in the case where power required by the steering assist motor rapidly increases when parking a vehicle in a garage or in a parking lot, for example, power from an auxiliary power source is added to power from the onboard battery (main power source) to thereby supply power corresponding to the power required by the steering assist motor.
An auxiliary power supply device according to PTL 1 includes a capacitor (auxiliary power source) that is connected to the main power source supplying power to a motor and that is capable of discharging to the motor, and a boosting circuit that boosts the voltage of the main power source and applies the resulting voltage to the capacitor. The auxiliary power supply device is controlled by a control device. When the supply of power from the main power source to the capacitor starts in a state that the voltage between the terminals of the capacitor is equal to or larger than the voltage of the main power source, the control device performs control to boost more the voltage applied by the boosting circuit, to the capacitor as the voltage between the terminals of the capacitor becomes larger.
With the auxiliary power supply device according to PTL 1, it is possible to suppress a backward current flowing from the auxiliary power source to the main power source upon the start of charging the auxiliary power source and to appropriately supply a forward current from the main power source to the auxiliary power source.